In cooling plants comprising heat rejecting heat exchangers, such as a gas cooler, gas cooler control may not succeed, when faults in the pressure being measured and/or faults in the temperature being measured at the outlet of the gas cooler exceeds normally expected values. FIG. 1 is a log P-h diagram showing how a read-out of a slightly too high pressure (upper limit of Δp) may result in the controller registering the pressure and/or the temperature at the outlet of the heat rejecting heat exchanger, for instance the gas cooler, to be at the optimal curve in point B in FIG. 1, while the physical situation, or the actual condition, is shown in point A in FIG. 1.
FIG. 1 illustrates how gas cooler control not succeeding, when faults in the pressure being measured and/or faults in the temperature being measured at the outlet of the gas cooler exceeds normally expected values affects the efficiency of the cooling plant. The continuous cycle marked by a continuous line represents the actual running cycle, while the cycle marked by the dashed line represents the cycle perceived by the controller. The two cycles require almost the same amount of energy per weight unit of refrigerant for compression, while the useful cooling capacity per weight unit of refrigerant is dramatically lower for the actual running cycle than for the cycle perceived by the controller.
Thus, while it is believed that the cooling plant is operated at near optimal conditions, it is in fact operated in a very energy inefficient manner. The problem is sometimes referred to as gas loop operation and may occur, where the isothermal lines are approximately horizontal in the log P-h diagram, as illustrated in FIG. 1.
WO2007022778 also describes the phenomena of ‘gas loop operation’, hereby incorporating entire WO2007022778 by reference. Reference is specifically, but not exclusively, made to page 5, lines 4-9, together with FIG. 4 of WO2007022778, said specific reference disclosing: In addition to the transcritical cooling cycle, FIG. 4 shows two isotherms 34, 36. It should be noted that a decrease of the gas cooler pressure at the point 3 moves the point 4 to the right by a large amount because of the low and almost horizontal slope of the isotherm 34 so that the available specific enthalpy for release in the evaporator decrease by a large amount. Since the specific enthalpy added by the compressor 14 decreases by a small amount, the resulting COP decreases by a large amount. Conversely, an increase of the gas cooler pressure at the point 3 moves the point 3 to the left by a small amount because of the steep slope of the isotherm 34 so that the available specific enthalpy for release in the evaporator increases by a small amount. Since the specific enthalpy added by the compressor 14 also increases by a small amount, the resulting COP hardly changes.
Gas loop operation reduces the cooling capacity to almost zero. It will result in the controller increasing the capacity of the compressor to 100%, and after a couple of minutes, the controller will increase the reference of the gas cooler.
DE 10 2006 019082 discloses a cooling apparatus for a vehicle. The apparatus includes a state detection unit (9, 9a, 13, 13a, 12, 14-16, 18, 19, 21, 22) for detecting a condition that an internal pressure of the refrigerant circuit exceeds a preset pressure. When it is detected that an internal pressure of the refrigerant exceeds the preset pressure, the apparatus controls a pressure reducing unit for reducing a pressure of the refrigerant on a low pressure side of a refrigeration cycle, when the condition is detected. When the state detecting unit detects the state, for example, starts the pressure reduction unit including a compressor of the cooling circuit so as to reduce the pressure on the low pressure side in the cooling circuit. DE 10 2006 019082 does not teach, and is not capable of, detecting a possible gas loop operational mode.